The invention relates to the fields of detecting and treating homozygous and heterozygous genetic deficiencies in ion transport, particularly alterations in nucleic acid molecules and proteins that give rise to various forms of pseudohypoaldosteronism type-1 (PHA1). More specifically, the invention provides compositions and methods for determining whether an individual is affected by or carriers a mutation a gene that encodes a protein involved in ion transport.
Background
Pseudohypoaldosteronism type I (PHA1) is a rare salt wasting disease characterized by an often fulminate presentation in the neonatal period with dehydration, hyponatremia, hyperkalaemia, metabolic acidosis, failure to thrive and weight loss despite normal renal glomerular filtration and adrenal function (Cheek, D. et al. Archives of Dis. in Childhood 33:252-256 (1958); Dillon, M. J. et al. Archives of Dis. in Childhood 55:427-434 (1980); Popow, C., et al. Acta Paediatr. Scand. 77:136-141 (1988); Speiser, P. W., Stoner, E. and New, M. I. Pseudohypoaldosteronism: a review and report of two new cases. In: Mechanisms and clinical aspects of steroid hormone resistance. (eds. Chrousos, G. P., Loriaux, D. T. and Lipsett, M. B.) 173-195. (Plenum Press, New York), 1986). PHA1 is suspected when these infants fail to respond to mineralocorticoids, and the diagnosis is supported by the finding of an elevated plasma aldosterone concentration and plasma renin activity (Dillon, M. J. et al. Archives of Dis. in Childhood 55:427-434 (1980); Popow, C., et al. Acta Paediatr. Scand. 77:136-141 (1988); Speiser, P. W., Stoner, E. and New, M. I. Pseudohypoaldosteronism: a review and report of two new cases. In: Mechanisms and clinical aspects of steroid hormone resistance. (eds. Chrousos, G. P., Loriaux, D. T. and Lipsett, M. B.) 173-195. (Plenum Press, New York), 1986). Treatment includes sodium chloride supplementation and treatment with an ion-binding resin or dialysis to reduce life-threatening hyperkalaemia (Cheek, D. et al. Archives of Dis. in Childhood 33:252-256 (1958); Dillon, M. J. et al. Archives of Dis. in Childhood 55:427-434 (1980); Popow, C., et al. Acta Paediatr. Scand. 77:136-141 (1988); Speiser, P. W., Stoner, E. and New, M. I. Pseudohypoaldosteronism: a review and report of two new cases. In: Mechanisms and clinical aspects of steroid hormone resistance. (eds. Chrousos, G. P., Loriaux, D. T. and Lipsett, M. B.) 173-195. (Plenum Press, New York), 1986; Donnell, G. N., et al. Am. J. Dis. Child. 97:813-828 (1959); Mathew, P. M., et al. Clinical Pediatrics. 1:58-60 (1993)). Death in the neonatal period is common if the diagnosis is not made.
PHA1 kindreds showing both autosomal recessive and dominant transmission have been described (Hanukoglu, A. J. Clin. Endocrin. and Metab. 73,936-944 (1991)). Cases in recessive kindreds typically show mineralocorticoid resistance in the kidney, sweat and salivary glands, and colonic mucosa (Speiser, P. W., Stoner, E. and New, M. I. Pseudohypoaldosteronism: a review and report of two new cases. In: Mechanisms and clinical aspects of steroid hormone resistance. (eds. Chrousos, G. P., Loriaux, D. T. and Lipsett, M. B.) 173-195. (Plenum Press, New York), 1986; Hanukoglu, A. J. Clin. Endocrin. and Metab. 73,936-944 (1991); Hanukoglu, A., et al. J. Pediatr. 125: 752-755 (1994); Hogg, R. J., et al. Pediatric Nephrology 5:205-210 (1991)); where measured, parents of these cases have had normal aldosterone and renin levels (Speiser, P. W., Stoner, E. and New, M. I. Pseudohypoaldosteronism: a review and report of two new cases. In: Mechanisms and clinical aspects of steroid hormone resistance. (eds. Chrousos, G. P., Loriaux, D. T. and Lipsett, M. B.) 173-195. (Plenum Press, New York), 1986; Hanukoglu, A. J. Clin. Endocrin. and Metab. 73,936-944 (1991)). In contrast, kindreds supporting dominant transmission have also been reported, and in some of these have been shown to have disease limited to the kidney (Speiser, P. W., Stoner, E. and New, M. I. Pseudohypoaldosteronism: a review and report of two new cases. In: Mechanisms and clinical aspects of steroid hormone resistance. (eds. Chrousos, G. P., Loriaux, D. T. and Lipsett, M. B.) 173-195. (Plenum Press, New York), 1986; Hanukoglu, A. J. Clin. Endocrin. and Metab. 73,936-944 (1991); Limal, J. M., et al. Lancet 1:51 (1978); Hanukoglu, A., et al. Lancet 1:1359 (1978)). Clinical signs and metabolic abnormalities of some patients improve in the first several years of life, allowing discontinuation of therapy (Cheek, D. et al. Archives of Dis. in Childhood 33:252-256 (1958); Dillon, M. J. et al. Archives of Dis. in Childhood 55:427-434 (1980); Speiser, P. W., Stoner, E. and New, M. I. Pseudohypoaldosteronism: a review and report of two new cases. In: Mechanisms and clinical aspects of steroid hormone resistance. (eds. Chrousos, G. P., Loriaux, D. T. and Lipsett, M. B.) 173-195. (Plenum Press, New York), 1986; Donnell, G. N., et al. Am. J. Dis. Child. 97:813-828 (1959); Hanukoglu, A. J. Clin. Endocrin. and Metab. 73,936-944 (1991)); it has been suggested that these patients are most often those with dominant transmission (Hanukoglu, A. J. Clin. Endocrin. and Metab. 73,936-944 (1991)).
The pathogenesis of this syndrome has not been elucidated. The triad of renal salt wasting, hyperkalaemia and failure to respond to mineralocorticoids is most compatible with a renal defect in the distal nephron (Cheek, D. et al. Archives of Dis. in Childhood 33:252-256 (1958); Rxc3x6sler, A. J. Clin. Endocrin. and Metab. 59:689-700 (1984)). While mineralocorticoid receptor levels in affected patients have been found to be low (Armanini, D. et al. N. Eng. J Med. 313:1178-1181 (1985); Kuhnle U. et al. J. Clin. Endocrin. and Metab. 70:638-641 (1990); Bosson, D. et al. Acta Endo. 113:S376-S381 (1986)) molecular studies have revealed no evidence for a primary defect in the mineralocorticoid receptor (Komesaroff, P. A., et al. J. Clin. Endocrin. and Metab. 79:27-31 (1994); Zennaro, M. C., et al. J. Clin. Endocrin. and Metab. 79:32-38 (1994)).
Electrogenic transepithelial sodium transport is the rate limiting step in sodium reabsorption in the distal nephron, the distal colon, salivary and sweat glands, and lung epithelia (Horisberger, J. D., et al. Cell Physiol. Biochem. 32:283-294 (1993)). In the kidney, this electrogenic sodium transport is positively regulated by aldosterone (Rossier, B. C. and Palmer, L. G. Mechanism of aldosterone action on sodium and potassium transport. In: The Kidney, physiology and pathophysiology (eds. Seldin, D. W. and Giebisch, G.) 1373-1409 (Raven Press, New York, 1992)) and is mediated by the amiloride-sensitive epithelial sodium channel (ENaC). This channel composed of at least three subunits of similar structure (Canessa, C. M., et al. Nature 361:467-470 (1993); Canessa, C. M. et al. Nature 367:463-467 (1994)), each with intracellular amino and carboxy termini, two transmembrane spanning domains, and a large extracellular loop. In humans, xcex1ENaC is present on human chromosome 12, while b and g are tightly linked on chromosome 1622.
Mutations resulting in constitutive activation of ENaC activity have been shown to cause an autosomal dominant form of hypertension, Liddle""s syndrome (Shimkets, R. A. et al. Cell 79:407-414 (1994); Hansson, J. H. et al. Nature Genetics 11:76-82 (1995); Hansson, J. H. et al. Proc. Nat. Acad. Sci. USA 92:11495-11499 (1995); Schild, L. et al. Proc. Natn. Acad. Sci. USA 92:5699-5703 (1995)), which is characterized by volume expansion, hypokalaemia and alkalosis.
The present invention provides compositions and methods that can be used to differentiate and diagnose ion transport deficiencies, particularly PHA1. The present invention further provides methods and compositions that can be used to identify heterozygous carriers for this disorder. Carriers, though not displaying severe clinical symptoms, nonetheless display mild to moderate pathologies.
The present invention is based, in part, on the identification of the role of the epithelial sodium channel (ENaC) in pathological condition associated with abnormal ion transport, particularly PHA1, hypokalaemic alkalosis, hypokalaemic acidosis and salt wasting. The present invention specifically provides the amino acid sequences of several human wild-type and altered variants of the ENaC proteins as well as the nucleotide sequence that encodes these variants. These proteins and nucleic acid molecules can be used in diagnosing ion transport disorders and in developing methods and agents for treating these pathologies.